The application of membranes in gas separation and pervaporation requires materials that are resistant to plasticizing feed streams. Removal of carbon dioxide from high-pressure natural gas exemplifies this type of separation. Plasticization results in unstable membrane performance and a loss in separation efficiency (increased methane losses). Polyimides have been identified as suitable materials for gas separation and pervaporation applications. To further improve the resistance of these materials to plasticization, methods of making crosslinkable polyimide polymers have been devised.
For example, U.S. Pat. Nos. 7,247,191; 6,932,859; and 6,755,900, which documents are incorporated by reference herein in their entireties, teach crosslinkable polymers and crosslinked hollow fiber membranes made from such crosslinkable polymers. These patents particularly describe a crosslinkable polyimide polymer. The crosslinkable polyimide polymer can be made by monoesterifying a polyimide that contains pendant carboxylic acid groups with a muli-functional alcohol crosslinking agent (e.g. diols, triols).
A crosslinked membrane can be made by transesterifying the monoesterified polyimide. More specifically, the crosslinkable polyimide polymer can be formed into crosslinkable membranes, which are then subjected to transesterification conditions in order to create covalent ester crosslinks between polyimide chains. Such membranes can be hollow fiber membranes or other types of membranes. Crosslinked hollow fiber membranes can be incorporated into a separation module. Other types of membranes for separation include flat sheet separation membranes, which can be used to create spirally wound membrane modules or flat stack permeators.
As further exemplary references of how crosslinkable polyimide polymers and membranes may be made, see Wind J. D., C. Staudt-Bickel, D. R. Paul, W. J. Koros, Solid-state covalent crosslinking of polyimide membranes for carbon dioxide plasticization reduction, Macromolecules, 2003, 36, 1882-1888; and Wind, J. D., S. M. Sirard, D. R. Paul, P. F. Green, K. P. Johnston, W. J. Koros, CO2-induced plasticization of polyimide membranes: Pseudo-equilibrium relationships between diffusion, sorption, and swelling, Macromolecules, 2003, 36, 6433-6441. These references are also incorporated by reference in their entireties.
A common attribute of these references is that they often provide for transesterification conditions wherein crosslinking occurs at relatively high temperatures, i.e., 200° C. or more. Alternatively, if lower temperatures are used, long periods of heating are required to achieve desired results in crosslinking For example, U.S. Pat. No. 6,932,859 provides that crosslinkable fibers are crosslinked at 150° C. for 25 hours. Commercially, requiring either high temperatures and/or long heating periods to cause crosslinking is undesirable.
It is known that the crosslinking or transesterification reactions can be acid-catalyzed. Incorporation of p-toluene sulfonic acid (p-TSA) into hollow fibers has been shown to lower the required temperature to make fibers insoluble in tetrahydrofuran (THF), a good solvent for the monoesterified polymer. See Wallace, D. W., Crosslinked hollow fiber membranes for natural gas purification and their manufacture from novel polymers, Ph.D. Dissertation, The University of Texas at Austin, 2004. However, the transport properties of these fibers were compromised by the inclusion of high concentrations of p-TSA (4 wt %).
Accordingly, there is a need to find methods for accommodating crosslinking at lower, more industrially-relevant temperatures and/or shorter heating times which do not compromise the separation capabilities of resulting crosslinked membranes. The present disclosure addresses this need.